In some wireless communication systems, single band array antennas are employed. However in many modern wireless communication systems network operators wish to provide services under existing mobile communication systems as well as emerging systems. In Europe GSM and DCS1800 systems currently coexist and there is a desire to operate emerging third generation systems (UMTS) in parallel with these systems. In North America network operators wish to operate AMPS/NADC, PCS and third generation systems in parallel.
As these systems operate within different frequency bands separate radiating elements are required for each band. To provide dedicated antennas for each system would require an unacceptably large number of antennas at each site. It is thus desirable to provide a compact antenna within a single structure capable of servicing all required frequency bands.
Base station antennas for cellular communication systems generally employ array antennas to allow control of the radiation pattern, particularly down tilt. Due to the narrow band nature of arrays it is desirable to provide an individual array for each frequency range. When antenna arrays are interleaved in a single antenna structure the radiating elements must be arranged within the physical geometrical limitations of each array whilst minimising undesirable electrical interactions between the radiating elements.
U.S. Pat. No. 6,211,841 discloses a dual band cellular base station antenna in which a high frequency band array of cross dipoles is interleaved with a low frequency band array of cross dipoles.
U.S. Pat. No. 6,333,720 discloses a dual polarized dual band antenna. An array of two low frequency band dipole squares are mounted above a ground plane. Dipole feeds angle outwardly from the centre of each group to form a dipole square. The high band radiating elements consist of an array of three cross dipoles. A cross dipole is provided at the centre of each dipole square and one cross dipole is provided between the dipole squares.
U.S. Pat. No. 4,434,425 discloses an arrangement of concentric dipole squares suitable for receiving radiation concentrated by a parabolic reflector antenna. The outer ring consists of vertically and horizontally polarised dipole pairs whereas the inner dipole square consists of dipole pairs having slant 45 polarization. The arrangement provides a common phase centre for receiving radiation from a parabolic reflector.
U.S. Pat. No. 4,555,708 discloses a satellite navigation antenna for producing radiation having circular polarization.
It is desirable to provide a multi-band antenna that is compact, easy to manufacture and inexpensive, having good isolation, appropriate beam width, minimal grating lobes and a good cross polarization ratio.
U.S. Pat. No. 6,317,099 and U.S. Pat. No. 6,285,666 describe a folded dipole antenna with a ground plane; and a conductor having a microstrip feed section extending adjacent the ground plane and spaced therefrom by a dielectric, a radiator input section, and at least one radiating section integrally formed with the radiator input section and the feed section. The radiating section includes first and second ends, a fed dipole and a passive dipole, the fed dipole being connected to the radiator input section, the passive dipole being disposed in spaced relation to the fed dipole to form a gap, the passive dipole being shorted to the fed dipole at the first and second ends.
The radiating section is driven with a feed which is not completely balanced. An unbalanced feed can lead to unbalanced currents on the dipole arms which can cause beam skew in the plane of polarization (vertical pattern for a v-pole antenna, horizontal pattern for a h-pole antenna, vertical and horizontal patterns for a slant pole antenna), increased cross-polar isolation in the far field and increased coupling between polarizations for a dual polarized antenna.
A stripline folded dipole antenna is described in U.S. Pat. No. 5,917,456. A disadvantage of a stripline arrangement is that a pair of ground planes is required, resulting in additional expense and bulk.
U.S. Pat. No. 4,837,529 describes a microstrip to coaxial side-launch transition. A microstrip transmission line is provided on a first side of a ground plane, and a coaxial transmission line is provided on a second side of the ground plane opposite to the first side of the ground plane. The coaxial transmission line has a central conductor directly soldered to the microstrip line. Direct soldering to the microstrip line has a number of disadvantages. Firstly, the integrity of the joint cannot be guaranteed. Secondly, it is necessary to construct the microstrip line from a metal which allows the solder to flow. The coaxial cylindrical conductor sleeve is also directly soldered to the ground plane. Direct soldering to the ground plane has the disadvantages given above, and also the further disadvantage that the ground plane will act as a large heat sink, requiring a large amount of heat to be applied during soldering.